Control
limit

The limits on a Control Chart such that, when data points fall outside
them, special causes of variation must be suspected. These are normally
three standard deviations either side of the mean.

Note: Control limits are calculated, not assigned. They are a
statistical point. Sometimes action limits, which are arbitrarily assigned
and outside of which some action is taken, get called 'control limits'.
Beware of such confusion.

There are different formulae for calculating control limits, depending
on the different type of Control Charts being used.